When radio communication is performed, presence of an interference radio wave is a cause of deterioration of the communication quality. Thus, countermeasures against the interference of the target radio wave are taken by monitoring the state of the radio wave interference. Examples of the countermeasures against the interference include controlling the output of a radio wave and changing the installation location of an apparatus outputting a radio wave. Effective countermeasures vary depending on the characteristics of the interference radio wave.
The characteristics of a radio wave depend on the specification defined by the corresponding radio standard, such as parameters used for the modulation or the method of controlling the radio wave. Thus, if the radio standards of radio waves being transmitted through the air are detected, it is expected that the interference of the target radio wave is effectively reduced. However, even if a radio wave is received and data is restored, there are cases in which the radio standard fails to be determined in detail from the data.
For example, it is difficult to distinguish Bluetooth (registered trademark) from Bluetooth Low Energy (BLE) from the restored data. However, some of the radio standards use different modulation parameter ranges. For example, Bluetooth using Gaussian frequency-shift keying (GFSK) uses a modulation index in the range between 0.28 and 0.35, and BLE uses a modulation index in the range between 0.45 and 0.55. An individual modulation index is defined as a product of a symbol period and a frequency shift amount.
In the case of radio standards that use different modulation index ranges for radio communication, these radio standards are distinguished from each other by detecting the modulation indexes from the respective radio waves received. Next, countermeasures against the interference may be taken on the basis of the determined radio standards. Thus, accurate estimation of an individual modulation index contributes to effective countermeasures against the interference. The accuracy of the estimation of a modulation index depends on the accuracy of the detection of an individual phase shift amount (corresponding to the accuracy of the detection of an individual frequency shift amount). However, in the case of a modulation method such as GFSK in which band limitation is performed on a baseband signal, the individual phase shift amount varies depending on the transmitted bit pattern.
For example, in the case of a setting in which the phase increases by +Δ per symbol period when the bit value is 1 and the phase decreases by −Δ per symbol period when the bit value is 0, if no band limitation is performed on a baseband signal, the individual phase shift amount is always Δ regardless of the bit pattern. In contrast, when the above band limitation is performed, the individual phase shift amount is not always Δ. For example, the phase shift amounts corresponding to the lower two bits of a bit pattern 001 are different from the phase shift amounts corresponding to the lower two bits of a bit pattern 101.
There has been proposed a method of estimating a modulation index when the above band limitation is performed. In this method, a phase shift amount Δ is estimated when a particular bit pattern (000 . . . , 111 . . . ) formed by the same bit values appears. When such a particular bit pattern appears, the maximum phase shift amount appears. Thus, even when the above band limitation is performed, as long as the bit pattern is accurately restored, the modulation index is accurately estimated (for example, see Japanese Laid-open Patent Publication No. 2003-23467).
However, it is not very frequent that the above particular bit patterns are included in wireless packets. When a bit pattern is restored, an error occurs in practice during the restoration processing. Thus, there is a risk that a phase shift amount that does not correspond to any of the particular bit patterns could be used for the estimation. When the frequency of the appearance of these particular bit patterns is low, the above risk could significantly affect the estimation accuracy. Namely, even if the above proposed method is used, the accuracy of estimation of the modulation index may be insufficient.